Diversification of DnaA dependency for DNA replication in cyanobacterial evolution

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• DNA replication initiation is crucial for cell viability and is typically regulated by the DnaA protein in prokaryotes.
• The absence of the dnaA gene in plastids and some symbiotic bacteria suggests the existence of DnaA-independent replication mechanisms, but their evolutionary origin remains unclear.

Purpose of the Study:

• To investigate the variability in DnaA dependency for DNA replication among different cyanobacterial species.
• To explore the implications of DnaA absence on DNA replication and cell viability in cyanobacteria.

Main Methods:

• Gene deletion experiments were performed on Synechococcus elongatus PCC 7942 to remove the dnaA gene.
• DNA replication sites and cell viability were analyzed in wild-type and dnaA disruptant strains of various cyanobacteria, including Synechocystis sp. PCC 6803 and Anabaena sp. PCC 7120.

Main Results:

• Deletion of the dnaA gene in Synechococcus elongatus PCC 7942 resulted in a shift of DNA replication initiation site from oriC due to episomal plasmid integration.
• dnaA disruptants of Synechococcus elongatus PCC 7942 exhibited higher viability during the stationary phase compared to wild-type cells.
• DNA replication and cell growth were unaffected by dnaA deletion in Synechocystis sp. PCC 6803 and Anabaena sp. PCC 7120.

Conclusions:

• Functional dependency on DnaA for DNA replication has been lost in certain nonsymbiotic cyanobacterial lineages.
• Cyanobacteria likely acquired DnaA-independent replication mechanisms prior to symbiosis, with an ancestral cyanobacterium potentially serving as the primary endosymbiont for plastid formation.